Components called “combiners” are used in radiofrequency systems for combining the power of two signals. For example, advanced amplifier architectures require this type of device in order to combine the amplified signals originating from multiple branches. This principle is notably used to produce linear amplifiers on the basis of amplifiers operating in the non-linear regime (LINC) and thereby enjoying better energy efficiency.
Generally, a combiner is a device making it possible for two (or more) signals to be combined into a single signal. According to the architecture of the combiner, it is desirable moreover to add isolation between the input ports, so as to limit the influence of each branch of the circuit on the others. Conventional combiners are embodied on the basis of discrete passive elements or of transmission lines, as illustrated in FIGS. 1a and 1b which show power combiners or dividers embodied in the form of transmission lines with isolation of the input ports and (without particular isolation).
In the context of mobile wireless telecommunications, such as mobile telephony, the dimensions of the circuits must be miniaturized. However, the traditional solutions occupy a sizable space and are difficult to miniaturize. For example, the solution using discrete passive components (inductors, capacitors) requires elements of high values that are difficult to integrate and exhibit high losses. Moreover, solutions using microwave transmission lines require long lines, since these depend on the electrical wavelengths which are typically of the order of a centimeter at frequencies of less than a few GHz. There therefore exists a significant need to find integratable, that is to say miniaturizable, solutions so as to allow the embodiment of numerous architectures requiring combinations of power, in particular RF, for applications of mobile telephony and portable wireless systems type.
In this context, several solutions have already been proposed. These solutions have in common the utilization of acoustic waves to produce combiner or splitter functions. These acoustic waves do indeed exhibit shorter wavelengths than the electrical wavelengths (of the order of a μm at frequencies of less than a few GHz), and therefore allow extreme miniaturization.
A first solution described in Japanese patent JP 60160719 “Surface acoustic wave power splitter”, 1985, proposes to produce a power divider (or power splitter) by using surface acoustic wave transducers arranged symmetrically along two distinct paths. The proposed structure is composed of a series of interdigitated-comb transducers, arranged on two acoustic paths as represented in FIG. 2, and linked to an input port and making it possible to convert the electrical signal into surface acoustic waves. These waves thereafter propagate to other transducers linked to the output ports and therefore make it possible to convey the electrical signal thereto. The symmetry makes it possible to ensure that one and the same power is transmitted to the two outputs. On the other hand, with a view to use as a combiner, this arrangement does not specifically ensure isolation between the ports 10 and 11: a signal received for example by the port 10 is transmitted to the port 12 which can in its turn re-emit waves on the lower acoustic path and transmit it to the port 11.
The Applicant has itself filed a patent application FR 10 53444, relating to a second solution using, in this instance, Lamb waves or bulk waves, represented respectively in FIGS. 3a and 3b. In addition to the fact that these solutions utilize various types of acoustic waves to transport the signal from the input ports to one and the same output port, they exhibit the benefit of providing isolation between the input ports. For the embodiment using Lamb waves, this is obtained by arranging along one and the same axis of propagation electrodes connected to the various ports in such a way that they exhibit well defined distances between them. Thus, for waves propagating from the emission electrodes to the reception electrodes, these distances make it possible to form constructive interference, and thus to aggregate their signals, and conversely to form destructive interference at the level of the input ports, thereby guaranteeing isolation between the various inputs. According to this principle, to succeed in interleaving three arrays of electrodes, it is necessary to resort to the embodiment of interconnections allowing the electrical tracks (for example of the output port) to straddle other electrical connections (for example those of the port 2). This principle may turn out to be complex to achieve. For the embodiment using bulk waves, the idea of using two acoustic paths, but this time vertical, is employed again. Along each acoustic path are arranged 3 series of transducers (piezoelectric layers sandwiched between two electrodes), linked, from top to bottom, to the output port, to one of the two input ports, and then to a floating port. Judicious choice of the thicknesses of the various layers of the stack succeeds in obtaining phase shifts between the waves causing constructive interference for those propagating from the input ports to the output ports, and destructive interference for those propagating from an input port to another output port, once again ensuring electrical isolation between the input ports. This embodiment requires a stack of three sets of piezoelectric layers, which, in terms of complexity of embodiment may surpass a longitudinal-coupling filter (CRF, for Coupled Resonator Filter), which is itself already a component that is so complex to produce that it is not at the present time proposed commercially as described in the article by C. Billard, N. Buffet, S. Joblot, A. Reinhardt, G. Parat, P. Bar, 200 mm manufacturing solution for coupled resonator filters, ESSDERC 2009.
It is thus apparent that patent FR 10 53444 proposes components ensuring a power combiner function, and moreover isolation of the input ports (or output ports in the case of the splitter) which are highly miniaturizable and integratable, but which may prove to be complex to produce and require the fine tuning of specific production methods.